The Texas in the Civil War Message Board

I was recently made aware that many of the men in Morgan's TX Cavalry Batallion/Regiment were originally in the 18th TX Cavalry before the incident at Arkansas Post. Is there any truth to this?

I specifically ask because I had ancestors in companies E and I of Morgan's Regiment. I now have a newly accessed listing of them being in companies E and I of the 18th Cavalry, not in Morgan's Regiment.